I'm guessing it's not an option for you because you've already specialized in a different genre. Your specialty genre is the one in which you first write 5 novels of that particular genre. And then ultimately, it's whatever genre in which you've penned the most books. So, the best way to write Masterpieces is to never specialize in one genre, but to only write 4 books of various genres until you fulfill the quota necessary to write a Masterpiece.